SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 60 of 74
Switch SW1 on the scheme logic module is used to select which of the two types of
fault detectors is used.
The normal setting is with SW1 in the left hand position which selects SOTF action
via the comparators, since this produces the faster tripping times when busbar VTs
are employed. However, when energising a line with a transformer and when a
large Zone 3 setting is required, there is a risk of transient operation of the Zone 3
comparators due to the magnetising inrush current of the transformer. Under these
conditions it is preferable to set SW1 to the right hand position, with SOTF action
via the level detectors.
The SOTF feature is enabled a short time after all three poles of the transmission
line are de-energised, as detected by the “pole dead” logic (see Figure 64),
signals from the level detectors. This time can be either 200ms (SW2 in right hand
position) or 110s (SW2 in left hand position). The normal setting is 200ms, since
this is long enough to prevent inadvertent operation of the SOTF feature during
transient dips in the ac voltage supply, but shorter than the shortest dead time of
the transmission line.
With an enable time of 200ms the SOTF feature has the additional advantage of
giving a fast trip when the breaker closes all 3 poles on to a fault near the far end
of the line (ie. beyond Zone 1 reach) with the breaker open at the far end. Without
this feature, only a delayed Zone 2 trip would normally be produced.
However, if the blocking scheme has been selected, circumstances are different.
If the breaker is closed on to a fault near the far end of the line, with the breaker
open at the far end, a fast aided trip is initiated by the Zone 2 comparators. Since
the SOTF is redundant under these conditions, it can be effectively disabled by
selecting the 110s time (SW2 to left-hand position). This time is greater than the
longest auto-reclose dead time and smaller than the line outage time required to fit
grounding clamps to the line. By disabling the SOTF feature for this period,
confusing indications can be avoided, while still retaining SOTF tripping for
closing on to grounded lines.
Once the SOTF feature has been enabled, it remains so for 240ms after the line
has been re-energised, or until a SOTF trip has been cleared. This period is long
enough for the synchronous polarising to be etablished if the line is healthy.
However, if a fault is present, 240ms is ample time for the fault to be detected.
While the SOTF feature is enabled the basic distance scheme trips and the carrier
aided scheme trips are disabled (but not the weak infeed feature of the permissive
overreach scheme).
When a fault is detected during the SOTF enable time, the SOTF logic outputs do
not reset until the fault is cleared.
If a SOTF trip occurs, the SOTF LED is illuminated and the auxiliary relay contact
98–1 closes. Also, a test point is provided on socket SK2 which enables
verification of the SOTF timers and logic, for commissioning and maintenance
purposes.
When busbar voltage transformers are used to supply the relay, the Zone 1 units
operate normally if the breaker is closed on to a bolted-on close-up three phase
fault and hence the SOTF feature is not needed for tripping. However, the SOTF
logic is still useful for blocking auto-reclose, for giving visual indication and remote
alarm. Since the “pole dead” level detectors cannot operate if busbar VTs are
used, the “breaker open” opto isolator is used (See Figure 64) to enable the SOTF
feature.
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